Chiral magnets such as MnSi, FeGe or Cu2OSeO3 exhibit a non-centrosymmetric lattice structure which lacks inversion symmetry. The resulting Dzyaloshinskii-Moriya interaction originating from weak spin-orbit coupling stabilizes smooth modulated magnetic textures, namely helices and skyrmions. In this thesis, we study the properties of helimagnets which are systems with a magnetic helix as ground state.
First, we examine the consequences of the helical texture for spin wave excitations, so-called helimagnons. We investigate magnon-focusing effects, i.e. magnon flow in very specific directions, which result from flat bands occurring in the helimagnon band structure when the momentum component perpendicular to the helix axis is large. We show that the softness of the Goldstone mode leads to a large dissipation even at very small frequencies cut off only by magnetocrystalline anisotropies or by a magnetic field. Finally, we discuss that dipolar interactions induce non-reciprocal behavior of the spectrum at finite fields and momenta, i.e. the spectrum is not symmetric under reversing the momentum anymore. We calculate the Brillouin light scattering cross section and compare it to experimental results obtained by N. Ogawa [1].
Then, we consider reorientation processes of the helix axis due to an applied magnetic field. We compare the results to magnetic force microscopy measurements in Cu2OSeO3 performed by P. Milde et al. [2]. Afterwards, we point out that the skyrmion lattice orientation has singular points, i.e. points where the orientation is not determined, as a function of the magnetic field direction which is a consequence of the Poincaré-Hopf theorem.
Afterwards, we turn to excitations in the form of the basic defects in helimagnets: disclinations and dislocations. Due to the lamellar nature of the helimagnetic texture, analogies to liquid crystals can often be used. We present an analytic parameterization of dislocations transferred from smectic liquid crystals and illustrate that dislocations carry a topological skyrmion charge. We examine dislocation motion in the presence of weak pinning due to random impurities. We derive a Thiele-Langevin equation for the dislocation position which effectively describes one dimensional motion. When reducing the system to two dimensions, this reveals ultra slow anomalous Sinai diffusion which may explain the very long time scales observed in several experiments [3,4].
Eventually, we present our work on domain walls in helimagnets. In magnetic force microscopy experiments performed by P. Schoenherr [5], we have identified three domain wall types. At small angles between the two domains, curvature walls appear. At intermediate angles, one can observe zig-zag disclination walls and at large angles, dislocation walls occur. We present analytical descriptions for curvature and dislocation walls, which we compare to micromagnetic simulation results obtained by J. Masell [5], and comment on the non-trivial topology of helimagnetic domain walls.
[1] N. Ogawa, L. Köhler, M. Garst, S. Toyoda, S. Seki, and Y. Tokura, In preparation (2019).
[2] P. Milde, E. Neuber, P. Ritzinger, L. Köhler, M. Garst, A. Bauer, C. Pfleiderer,
H. Berger, and L. M. Eng, In preparation (2019).
[3] A. Dussaux, P. Schoenherr, K. Koumpouras, J. Chico, K. Chang, L. Lorenzelli,
N. Kanazawa, Y. Tokura, M. Garst, A. Bergman, C. L. Degen, and D. Meier, Nature
Communications 7, 12430 (2016).
[4] A. Bauer, A. Chacon, M. Wagner, M. Halder, R. Georgii, A. Rosch, C. Pfleiderer, and M. Garst, Physical Review B 95, 024429 (2017).
[5] P. Schoenherr, J. Müller, L. Köhler, A. Rosch, N. Kanazawa, Y. Tokura, M. Garst,
and D. Meier, Nature Physics 14, 465 (2018).:Introduction
1. Introduction to chiral magnets
1.1. Helimagnets
1.1.1. Magnetic phase diagram of chiral magnets
1.2. Skyrmions
1.2.1. Topology
1.2.2. Magnetic skyrmions
1.2.3. Skyrmion motion
1.2.4. Emergent electrodynamics
1.3. Model for chiral magnets
2. Spin waves in helimagnets
2.1. Linear spin wave theory for helimagnons
2.1.1. Fluctuations in the harmonic approximation
2.1.2. Spectrum at small momenta and fields
2.1.3. Frequency broadening from Gilbert damping
2.2. Magnon-focusing effects
2.3. Enhanced local dissipation
2.3.1. Global static susceptibility in the limit k, k' → 0
2.3.2. Local damping
2.4. Non-reciprocity
2.4.1. Non-reciprocity of the spectrum
2.4.2. Brillouin light scattering cross section
3. Orientation of magnetic order
3.1. Helix reorientation transition in MnSi
3.1.1. Effective Landau potential for the helix pitch
3.1.2. Experimental results
3.2. Helix reorientation in Cu2OSeO3
3.3. Skyrmion lattice orientation
4. Disclinations and dislocations
4.1. Liquid crystals
4.1.1. Types of liquid crystals
4.1.2. Energetics of liquid crystals
4.2. Disclinations
4.2.1. Elasticity theory for disclinations
4.3. Dislocations
4.3.1. Volterra process and Burgers vector
4.3.2. Elasticity theory for dislocations
4.3.3. Mermin-Ho relation in helimagnets
4.3.4. Topological skyrmion charge
5. Dislocation motion
5.1. Thiele approach for one helimagnetic dislocation
5.1.1. Motion in the presence of pinning
5.1.2. Corrections from elastic deformations
5.2. Dislocation diffusion
5.2.1. Sinai diffusion and toy model simulations
5.2.2. Susceptibility with Sinai diffusion
5.2.3. Dislocation string
6. Domain walls
6.1. Experimental and numerical methods
6.2. Domain wall types in helimagnets
6.3. Energetics of helimagnetic domain walls
6.3.1. Curvature wall
6.3.2. Dislocation wall
6.4. Topological domain wall structures
7. Discussion and outlook
Appendix
A. Details on helimagnons
B. Formalism of linear-spin wave theory in helimagnets
C. Deviations from the helix
Bibliography
List of Figures
Index
Danksagung
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:73766 |
| Date | 08 February 2021 |
| Creators | Köhler, Laura |
| Contributors | Garst, Markus, Vojta, Matthias, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | info:eu-repo/grantAgreement/Deutsche Forschungsgemeinschaft/SSF 1143/247310070//Korrelierter Magnetismus: Von Frustration zu Topologie |
Page generated in 0.0027 seconds